Why is available space less then free?
 

Gnome-system-monitor. "File systems" tab. There are free and available columns. And available is always less then free? Why?

Free means that much is free but available means that much is available for usage. We can use the available space but not the free space.

So some MB should always stay free?
And why doesn't "df" show this info?

Free space is space that's not used to store any meaningful information.

Available space is space you can use to store your files.

The difference between both numbers can be because of many things. Your user might have a disk quota assigned, and some file systems reserve some disk space, so no one can use it. This is done to guarantee that the root user will have some available space for diagnosis and administrative purposes if needed. You can set this reserved space when formatting the volume or at any time using tune2fs.

However that's an fs specific feature. ext2/3/4 work this way. Other fs's might not have this feature, or it might work in a different way.

i92guboj, why doesn't "df" show this info?

What info are you exactly talking about?

The purpose of df is to show you info about how much space is occuppied by your files, and how much space is there still left for you to store files into your disks.

df is not tied to any given fs, not even to a given OS, because it has to work on a number of them. df is a pretty simple tool that just formats on a nice table the info that the fs driver supplies. Nothing else.